In 2014, the Ebola virus erupted in West Africa. The epidemic remains a humanitarian crisis of epic proportion. More than 20,000 people likely have been infected by the virus, which so far has killed at least 7,842, the World Health Organization reports.

The epidemic began quietly in rural southeastern Guinea in the last days of 2013. The Ebola virus then raged through Guinea, Liberia and Sierra Leone. It made forays into Nigeria and Mali, killing more than a third of those it infected, including doctors and caregivers. In its wake, Ebola left behind thousands of orphans.

This has been the biggest and most challenging Ebola outbreak since the virus was identified in 1976, according to WHO.

More than 20 outbreaks in other parts of Africa have been much more easily contained, said Dr. Tom Kenyon, who heads the Center for Global Health at the U.S. Centers for Disease Control and Prevention in Atlanta, Georgia.

Where those earlier outbreaks have occurred, "the local public knows it is Ebola and takes measures to stop it," Kenyon said. "They stop touching sick people. They stop touching dead bodies and bury them safely. They wash their hands with bleach solutions and don't touch one another. That will stop the outbreak."

FILE - Nowa Paye, 9, is led to an ambulance after showing signs of Ebola infection in Freeman Reserve, a village 30 miles north of Monrovia, Liberia.

Unfamiliar territory

But in West Africa, where the virus previously had not surfaced, even doctors were caught off guard.

"There’s lots of belief that it’s a curse, that a witch came and caused these things to happen to people," said Leisha Nolen, a physician in the CDC’s Epidemic Intelligence Service.

As a disease detective, she has gathered research on the epidemic in both Liberia and Sierra Leone. There and in Guinea, she said, civil wars left residents fearful of outsiders and unwilling to work with them or to change practices that spread the disease.

Unlike in the remote areas of Central Africa, people in the three affected countries used to travel freely between rural areas and major cities, as well as across borders.

"Movement is actually a big problem in this outbreak in that people continue to move and continue to expose new populations to the infection," she said in September.

All three countries have since established checkpoints to take temperatures and screen for the Ebola virus.

Travel bands urged, discouraged

Some airlines suspended flights to West Africa, amid calls for travel bans as a few cases were reported in the United States and Spain. But health officials and the United Nations opposed such measures on the grounds that they would further isolate the countries, depriving them of needed medical personnel and supplies and increasing the likelihood of the disease spreading.

As U.N. Secretary-General Ban Ki-moon said at a November 21 news briefing, "We need to avoid travel bans, discrimination against health workers and other steps that would isolate countries when they need help most."

Guinea’s Ministry of Health first formally acknowledged the current outbreak in March.

On March 31, the medical charity Doctors Without Borders warned that the outbreak was "unprecedented" in its geographic spread. In June, the organization's director of operations, Dr. Bart Janssens, issued a stronger statement, saying the outbreak was "out of control" and required a massive infusion of international health workers, resources and equipment to curb its spread.

In August, WHO declared the outbreak "a public health emergency of international concern."

Increased support from US, elsewhere

In September, President Barack Obama announced the United States would ramp up its efforts to fight the epidemic, beefing up humanitarian aid, support for awareness training and medical missions. The Defense Department deployed nearly 3,000 troops to West Africa to speed the delivery of medical personnel and supplies and to build treatment centers in Liberia.

By mid-December, WHO’s chief, Dr. Margaret Chan, reported signs of hope in some areas – and new outbreaks in others, especially in Sierra Leone.

"We must maintain our vigilance. Complacency would be our enemy," Chan had said a month earlier, noting then that the rate of infection for new cases had slowed. "… We have been successful in bending the curve a bit. But we need to continue to do more to get to zero."